Generally, commercial wind turbines can be divided into fixed speed turbines and variable speed turbines.
For a fixed speed turbine, the power production is only optimized for one specific wind speed, whereas for a variable speed turbine, optimal power output can be achieved for a wider range of wind speeds.
Since the late 1990's, the main part of the implemented larger wind turbines have been variable speed turbines, which require more complex electrical systems than fixed speed turbines. Also, newer grid requirements add to the complexity of the electrical systems of a modern wind turbine.
A full range variable speed turbine can be achieved by connecting the stator of the generator of the wind turbine to the grid through an AC-AC converter (such as a back-to-back converter or a matrix converter) changing the electrical output from the generator output frequency to the nominal grid frequency. An advantage of such a system is that, at least in principle, the full speed range from zero RPM to the maximum speed allowed for safety reasons can be used for production of electrical power. A disadvantage, on the other hand, is that the AC-AC converter must be rated to handle the full output power of the turbine.
In order to reduce the requirements of the AC-AC converter, it is known to use limited range variable speed systems, such as doubly-fed induction generator (DFIG) systems.
In a standard DFIG system, the stator is connected directly to the grid, normally via a transformer, while the rotor is connected to the grid via slip rings and an AC-AC converter. The limitations on the speed range of the system depend on the AC-AC converter, since the amount of power through the rotor is proportional to the difference between the electrical rotor speed and the synchronous speed (stator field speed) of the generator. Here and in the following, the term “electrical rotor speed” refers to the product of the mechanical rotor speed and the number of pairs of poles in the rotor.
The DFIG system suffers from a well-known disadvantage, namely poorly damping of oscillations within the flux dynamics due to cross coupling between active and reactive currents, which makes the system potentially unstable under certain circumstances and complicates the work of the rotor current controller, whose main task is to limit the rotor current in order to prevent the AC-AC converter from tripping or breaking down. It should be noted, that although the rotor current controller is in fact a current controller, the output control signal from the controller can comprise one or more voltages as well as currents, since the rotor currents can very well be controlled indirectly by controlling the rotor voltages.
A traditional way of avoiding oscillations is to reduce the bandwidth of the rotor current controller by a certain factor compared to the bandwidth of the power loops within the system, which however causes the rotor current controller to react slower to changes in the grid conditions. Furthermore, the controller performance (bandwidth, rise time etc.) depends on the rotor speed, which in best case results in high qualification and test costs and in worst case can cause controller instability and hardware failure.
The paper: “Control of a Doubly-Fed Induction Generator for Wind Energy Conversion Systems” (F. Poitiers, M. Machmoum, R. le Doeuff and M. E. Zaim, Ecole Polytechnique de l' Université de Nantes, Saint Nazaire, France) describes the control of electrical power exchanged between the stator and the grid by controlling active and reactive power, respectively. A model of a DFIG system is disclosed and a block diagram of the power control is shown. However, it is stated in the paper that the cross coupling between active and reactive power is of small influence and can be neglected, which is not in accordance with the common experience of most people working with modern wind turbines.
In the paper: “Stability Analysis of Field Oriented Doubly-Fed Induction Machine Drive Based on Computer Simulation” (Song Wang and Yunshi Ding, Department of Electrical Drive Automation, Automation Research Institute of Ministry of Metallurgical Industry, Beijing, People's Republic of China, 1993), computer simulations of different operational characteristics of a model of a DFIG system is presented. The simulation model introduces a voltage feed-forward function that weakens the cross coupling between the currents along the d- and q-axes, respectively. An actual compensation method, however, is not disclosed.
The Ph.D. Thesis: “Analysis, Modeling and Control of Doubly-Fed Induction Generators for Wind Turbines” (Andreas Petersson, Department of Electric Power Engineering, Chalmers University of Technology, Göteborg, Sweden, 2003) discloses a range of different methods for cross coupling compensation, all dealing with the oscillation and bandwidth problems. The methods mentioned all suffer from disadvantages, such as necessary differentiation of measured signals, reduction of rotor current controller bandwidth, need of extra inverter hardware etc.
International patent application WO 2004/098261 discloses a variable speed wind turbine with a DFIG, a rotor current controller of which regulates the flux-producing rotor current in order to secure that the wind turbine can stay connected (“ride through”) in the case of voltage fluctuations and/or transients on the utility grid. This system, however, does not provide any solution to the above-mentioned problem of potential instability of the system due to oscillations in the flux dynamics.
Since oscillation problems in the drive trains of wind turbines have been known for several years, different solutions have been suggested to solve or at least reduce these problems. One system for controlling drive train damping features based on the generator rotor speed in a wind turbine with a DFIG is disclosed in International patent application WO 99/07996. Here, a torque command generator with a feed-forward damping filter is used to generate a commanded torque in response to the measured generator rotor speed, the torque being controlled via the rotor current by a torque controller, and a PID controller performs pitch regulation based on the difference between the actual generator rotor speed and a target generator rotor speed.
An objective of the present invention is to provide a Doubly-Fed Induction Generator (DFIG) system in which the above-discussed possible oscillations may be controlled or substantially avoided in an improved manner.
Another objective of the present invention is to provide a DFIG system with improved ability to control the quality of the power output, which is necessary to meet the newest grid demands.
Also, it is an objective of the present invention to provide a compensated DFIG system that requires neither differentiation of measured signals nor extra hardware compared to traditional DFIG systems.